In laser diodes, a photodiode is often used to measure the emitted light and feed this information back to the current source feeding the laser diode. The feedback signal is used to adapt the operating current in a way that enables the laser light to stay within a desired dynamic window. This arrangement is called a laser diode with a monitoring photodiode.
In the case of edge-emitting laser diodes, it is known to monitor the light output of the diode by placing a photodiode directly on the backside laser beam. This is possible because an edge-emitting laser diode emits light through both its front and rear mirrors. The monitoring photodiode is placed adjacent the rear mirror and does not disturb the useful emission from the front mirror.
Recently, there has been a trend in the art to replace edge-emitting diodes by vertical cavity surface emitting laser diodes (VCSELs). In a VCSEL, the rear mirror is embedded inside the laser die and a result the only accessible light is that emitted through the front mirror at the top of the die. Some means must be provided of tapping off some of the useful light off for use as a monitor signal. A traditional method of coupling light from the front surface of the VCSEL to the monitoring photodiode is to use two mirrors, one placed in front of the VCSEL and the other in front of the photodiode, each at a 45.degree. angle to the beam. The mirror in front of the VCSEL is semitransparent. This arrangement has the disadvantage of being complicated to assemble and results in a device with an inherent tracking error due to the use of discrete passive parts, i.e. the mirrors and the means to fix the mirrors.